Coatings may be used for example for providing a protective coating for a base metal, usually a ferrous metal, of ladles used for molten metal melts, such as aluminium, magnesium or zinc based melts. In order to be effective, the coating must be fairly abrasion resistant, be sufficiently thick and adequately adherent to the surface of the base metal. It is also possible to use coatings for achieving other functional purposes, such as when using reactive coatings.
Thermal analysis is a technique monitoring variations in temperature change of certain molten substances during solidification to be able to determine the microstructure and hence properties of the substances in solid form. This is accomplished by taking a sample from the melt, transferring it into a sample vessel and recording and evaluating a time-dependent temperature change in the sample during solidification, by means of temperature responsive means, such as thermocouples or other devices known in the art.
WO 86/01755 discloses a method for producing compacted graphite cast iron by using thermal analysis. A sample is taken from a bath of molten cast iron and this sample is permitted to solidify during 0.5 to 10 minutes. The temperature is recorded simultaneously by two temperature responsive means, one of which is arranged in the centre of the sample and the other in the immediate vicinity of the vessel wall. So-called cooling curves representing the temperature of the iron sample as a function of time are recorded for each of the two temperature responsive means. According to this document it is then possible to determine the necessary amount of structure-modifying agents that must be added to the melt in order to obtain the desired microstructure.
Moreover, WO 92/06809 discloses a method for production of compacted graphite iron using thermal analysis. A melt of a suitable composition and structure is prepared and a modifying agent, such as magnesium, is added to the melt in an amount which is expected to be sufficient to produce compacted graphite iron. A sample is then extracted from the melt in a sample vessel, the wall of which is coated on its inside surface with a layer consisting of a material which will react with dissolved elementary magnesium present in the vicinity of said wall. Two thermocouples, one placed in the centre of the melt and the other in the vicinity of the vessel wall, are used to record the temperatures as a function of time during solidification of the sample melt, thereby giving information of phase transformations during solidification. The recorded temperature curves are thus used to determine if the magnesium content and the inoculation of the melt, from which the sample has been taken, has to be modified in order to achieve the desired phase transformations of the melt when producing a casting thereof in order to achieve the desired structure of graphite in the casting.
The fact that the vessel wall is coated with a layer consisting of a material which will react with dissolved elementary magnesium, and the arrangement of the thermocouples in the sample, gives two separate temperature readings giving information regarding the magnesium content of the melt. The thermocouple in the vicinity of the vessel wall records a temperature curve of a melt with a predetermined lower level of dissolved elementary magnesium (determined by the selected coating) than the thermocouple arranged in the centre of the sample melt and thus the structural changes of said composition. Thus, by comparing the readings of the two thermocouples, a more accurate measurement of the magnesium content of sample melt and thus the solidification properties of the melt from which the sample has been taken, is achieved.
The use of a coating which reduces the content of magnesium in the vicinity of the vessel wall of a sampling device may also advantageously be used when conducting thermal analysis of other metal melts, such as ductile cast iron (comprising nodular graphite particles) or grey iron (comprising flake/lamellar graphite particles).
WO 97/35184 discloses a sampling vessel where a part of the inner surface is coated with a reactive layer, the layer being of a substance which will lower the concentration of dissolved elementary magnesium to increase the accuracy of the solidification structure predictions of molten cast iron. According to this document, one temperature responsive means is placed in the centre of the sample vessel, another temperature responsive means is placed close to the inner surface of the sample vessel which has been coated with the reactive layer and a third temperature responsive means close to the inner surface of the sample vessel which has not been coated with the reactive layer. Each of the temperature responsive means are used to record the temperature of the melt during the solidification process as a function of time to obtain temperatures which are to be evaluated to determine the evolution of the solidification process. The reactive coating comprises 0-5 sulphur, 0-10% oxides of silicon, manganese or iron, and/or 0-0.5% oxides of potassium and sodium.
It has now been found that the previously known coatings for sampling devices intended to be used for thermal analysis does not always work as intended and may be difficult to give sufficiently accurate readings during thermal analysis, especially in the case of thermal analysis of ductile iron. Thus, there is still room for further improvements of previously known coatings.